The present invention relates to a method and apparatus for repairing efficiently white-spot defects on a photo-mask pattern.
Defects created on a photo-mask fall into two categories, a residual defect (opaque-spot defect) which is an opaque material left at a portion of the mask that must be transparent, and a transparent defect (white-spot defect) which is the absence of an opaque material at a portion of the mask that must be black. These defects impair the yield of product of semiconductor devices such as LSIs, and their repairing process greatly affects the productivity of the devices. Therefore, the repairing process must be as simple as possible, and must be done in a minimal time. Among defects created on a photo-mask, the repairing process for residual defects has been simplified considerably through the introduction of laser technology as disclosed in, for example, Japanese Patent Publication No. 52-9508. Whereas, transparent defects are generally repaired by the "lift-off" method as disclosed in, for example, P.H. Singer: Photomask and Reticle Repair; Equipment and Techniques: Semiconductor International 1982/8 pp. 79-90, pp. 86-88 or U.S. Pat. No. 4,444,801.
The lift-off method includes the following process steps.
(1) Application of positive-type photoresist on a pattern of photomask which includes transparent defect(s).
(2) Exposure of only the defective portion to the light by the partial exposing method.
(3) Opening of a window at the defective portion of the photoresist film by the developing process.
(4) Formation of a metallic film by sputtering over the photoresist film at the defective portion and its periphery, or on the entire surface of the photo-mask.
(5) Removal of the photoresist and unnecessary metallic film on the photoresist film, so that only the transparent defect(s) are covered by the metallic film.
The lift-off method includes numerous steps of process as described above, leaving a technological problem from the viewpoint of productivity.
Recently, there has been reported a method of metal deposition from the organic metal solution through the irradiation of laser, offered in the field other than photo-mask technology. (Refer to an article entitled, "Metal Deposition from Organic Metal Solution by Irradiation of Laser", p. 202, in the proceedings of the 30th Annual Conventional Record of Joint Applied Physics Societies, Spring 1983.)
In this method, as shown in FIG. 1, benzene solution 1 including Cr-complex or Mo-complex is filled in a cell 3 having two glass windows 2a and 2b, and a laser beam 4 is focused by an objective lens 5 onto the interior surface of the glass window 2a, and then metal 6 is deposited.
The inventors of the present invention have attempted to apply this technique to the repairing of defects on the photo-mask pattern. However, Cr-complex or Mo-complex used in this process is very unstable, and apt to create oxide when exposed to oxygen in the atmosphere, posing a handling problem. Moreover, deposited metal is apt to form a gathering of particles. Therefore, there are still technical problems to be solved before applying this method to the repairing of defects on the photo-mask pattern.
In addition to the above methods, there has been reported a technique of repairing the transparent defects by resolving and depositing a metal from organic metallic gases by means of laser beam projection. Such a technique has been disclosed for example in D. J. Ehrlich et al's article: Laser Microphotochemistry for use in Solid State Electronics; IEEE Vol. QE-16 No. 11, pp. 1233-1243 (1980, 11). However, dimethyl cadmium referred to in the article is also apt to create oxide when exposed to oxygen in the atmosphere. Since dimethyl cadmium is difficult to be treated, there are technical problems to be solved before this method can become practical. Further, there is no disclosure about a repairing apparatus in the article.